* 50,000 gems - You can literally play any song in any level for this and you can repeat any songs too so it's just a case of grinding it out. The higher level songs on expert will give more gems if you want it faster, but those are harder.

* Complete all Red Songs - This is easier than it sounds. I was eating my dinner when I did this. You cannot fail songs so you just need to complete it. I just turned on the song and I was in the background somewhere so the game didn't pause on me. Of course, this method doesn't gain you many gems, so it's probably best if you just play them properly for more gems. The easy version has only 3 songs in each section as opposed to 6 on expert.

I highly recommend just starting the game on expert to save yourself replaying stuff later. You can always pop down to easy for some of the more awkward achievements later.